1. Field of Invention
This invention relates to electroluminescent displays, and more particularly to an electroluminescent display having reversible polarity for creating images on an electroluminescent panel.
2. Description of the Prior Art
Electroluminescent displays represent a class of flat panel displays that are used in a wide variety of applications. For example, the displays are currently used in military systems, elevators and hospital monitoring equipment.
Electroluminescent displays generally include an electroluminescent layer such as a ZnS phosphor doped with an activator such as Mn. The electroluminescent layer is placed between two dielectric layers. A first series of parallel and longitudinal electrodes adjoin the first dielectric layer and a second series of parallel and longitudinal electrodes adjoin the second dielectric layer in an orthogonal orientation with respect to the first series of electrodes.
The first series of electrodes may be referred to as row electrodes and the row electrodes may be constructed of aluminum. The second series of electrodes may be referred to as column electrodes. The column electrodes are typically transparent and made of indium-tin oxide.
An intersection of the first series and second series of electrodes defines a picture element referred to as a pixel. The resolution of the electroluminescent display is determined from the number of pixels.
The electroluminescent displays operate by applying a voltage across the electroluminescent layer via the first series and second series of electrodes. Each pixel within the electroluminescent layer will emit light when a sufficient voltage is present between the electrodes which correspond to the pixel. The luminescence of the particular pixel will be determined from the magnitude of the voltage across the pixel.
Electroluminescent displays may suffer from a problem referred to as a `latent image` or `retained image` phenomenon. This phenomenon results in smearing and ghost images wherein an image which has been displayed for a long period of time may be burned into the display (i.e. the image is apparent to varying degrees even though it is not electrically written on the display). Accordingly, this problem is most severe in areas of the electroluminescent layer which are subject to the greatest use. These images may appear only after a few hours or several days or months depending upon the technology and electronic voltage drive scheme utilized.
It is believed that the basic cause of this phenomenon is the occurrence of sulfur vacancies within the Mn-doped ZnS phosphor. These sulfur vacancies diffuse in a non-uniform manner within the phosphor with the passage of time and thereby change the electrostatics of the device.
This theory is supported by the fact that the occurrence of a latent image is greatly dependent upon the electronic voltage drive scheme. It appears that the latent image phenomenon is a result of the pixels having a voltage-time average that is non-zero when averaged over several scans through the model. The non-zero voltage-time average causes an asymmetrical charge distribution to be built up over time and possibly a spatially preferential accumulation of sulfur vacancies within the phosphor.
One approach to reduce the severity of the latent image phenomenon is to utilize a symmetric voltage drive scheme. Symmetric voltage drive schemes are well known in the art and operate by first generating a plurality of positive voltage pulses followed by a plurality of negative voltage pulses which are equal in magnitude to the corresponding positive voltage pulses. The average electric field within the ZnS phosphor approaches zero when a symmetric waveform is used to drive the electroluminescent display and there is no spatially preferential accumulation of sulfur vacancies within the phosphor.
However, the use of a symmetric voltage drive scheme is undesirable inasmuch as pixel brightness is reduced up to 50% as compared to the use of an asymmetrical voltage drive scheme. This reduction in brightness of the electroluminescent display is unacceptable when high ambient viewability is required. It has also been noticed that a symmetric voltage drive scheme may cause a ghosting phenomenon in certain display modes such as scrolling characters across a display. In addition, the response time of an electroluminescent display which is driven by a symmetric voltage drive scheme is slower than an electroluminescent display driven by an asymmetrically driven electroluminescent display.
Asymmetrical voltage drive schemes are also well known in the art. These voltage drive schemes operate by generating a first refresh voltage pulse which is followed by a plurality of write voltage pulses corresponding to a first write cycle. The first write cycle may be followed by a second refresh pulse and a second write cycle and the pattern is repeated. The refresh pulses have a polarity which is opposite that of the write pulses. The use of asymmetrical voltage drive schemes offers the advantages of faster response time and a brighter electroluminescent display without the ghosting phenomenon in certain display modes.
Despite the advantages of an asymmetrical drive scheme, the magnitude of the opposite polarity drives are not equal and a charge may accumulate at an interface of the electroluminescent layer and a dielectric layer resulting in the appearance of ghost images within the electroluminescent display.